An acrylic rubber has high heat resistance and good oil resistance, and therefore is widely used in automobile and related fields. However, in fields of sealing material, hose material, vibration insulating material, tube material, belt material and boot material, more enhanced heat aging resistance and reduced compression set are recently eagerly desired. Further, an acrylic rubber has a problem in storage stability, namely in the fact that the viscosity of an uncrosslinked acrylic rubber composition undesirably increases during storage and the processability thereof is reduced.
In view of the above-mentioned beneficial properties, an acrylic rubber is widely used as a colored material. In the field of a colored material, a white filler such as synthetic silica, i.e., white carbon, is usually incorporated in a rubber composition. However, a white filler particle has a small specific surface area and a poor interaction with a rubber polymer, and thus, an acrylic rubber composition having a white filler incorporated therein has poor mechanical properties. Therefore, a crosslinked object of a white filler-incorporated acrylic rubber having improved mechanical properties is also desired.
To provide a crosslinked rubber of white filler-incorporated acrylic rubber having improved mechanical strength, incorporation of silica having a pH value in the range of 6.5 to 8.5 and a specific surface area of at least about 150 m2/g has been proposed (Japanese Unexamined Patent Publication H8-109302). However, the crosslinked object of the silica-incorporated acrylic rubber composition tends to have a rather large compression set.
To provide a white filler-incorporated acrylic rubber composition having enhanced storage durability and giving a crosslinked rubber having good mechanical properties, there has been proposed an acrylic rubber composition comprising a halogen-containing acrylic rubber, a triazine thiol crosslinking agent, a dithiocarbamic acid derivative, hydrotalcite, an aromatic carboxylic acid, a white filler having a pH value in the range of 2 to 10 and a silane coupling agent (Japanese Unexamined Patent Publication H10-53684). However, the storage stability of the acrylic rubber composition is not improved to the desired extent and the compression set of the crosslinked rubber is not sufficiently reduced.